From BR Bullpen
The leadoff hitter is the hitter whose name appears first in the batting order. His job is to reach base by any means possible, in order to provide the team's best power hitters - who usually bat from the third to the fifth spot in the order - the chance to drive him in.
Traditionally, teams placed players likely to get on base at the top of the order - people such as John McGraw or Jimmy Ryan. This trend began to change in the 1950s and 1960s with increased focus on stealing bases. Modern managers have tended to use some of their speediest players in the leadoff spot, to allow that player to steal a base and place himself in scoring position without help of a teammate. While this is sometimes a good strategy, sabermetrically-inclined managers will prefer to use a slower player with an ability to draw walks to a speedster with a low on-base percentage, as this will lead to more scoring in the long run. For example, speedy outfielder Omar Moreno was continually used a leadoff hitter during his career in the 1970s and 1980s, but his lack of walks actually made him a very poor choice for the task; in contrast, his contemporary Brian Downing, a slow outfielder converted from catching, was actually an excellent leadoff hitter in spite of his lack of speed because of the large number of walks he would draw.
 Further Reading
- Herman O. Krabbenhoft: Leadoff Batters of Major League Baseball: Complete Statistics 1900-2005, McFarland, Jefferson, NC, 2006.